Hydraulic Lines
I remember seeing DC-10s parked in large numbers while the FAA and NTSB were sorting out what to do. The San Diego 727 collision with a C-172 first hit by the nosewheel then hit the leading edge and took out all three hydraulic lines routed next to one another. The Sioux City DC-10 lost hydraulics when a piece of the #2 turbine disk went through the elevator.
Hard lessons.